The present invention generally relates to an optical disk drive and, more particularly, to a sum signal processing device for use in an optical disk drive for detecting a sum signal which is a groove-crossing signal used to detect a relative position of a beam spot for recording and reproducing information from a medium and the medium and a relative velocity in the radial direction of the medium.
An optical disk drive uses a light beam for recording and reproducing information from a medium. A prerequisite with such a light beam is that it forms an extremely small spot on the surface of the medium and thereby irradiates a desired extremely small area of the latter. Accurate control over the position of the light beam is critical for increasing the storage capacity available of an optical disk drive. Even though the beam spot may be accurately positioned, the disk drive is not satisfactory if the positioning operation is slow. Ideally, therefore, an optical disk has to position the beam spot accurately and rapidly. A predominant implementation for the easy control over the beam spot is a medium having concentric or spiral guide grooves thereon. Specifically, the beam spot is held in a predetermined position relative to the guide grooves so that it may follow a desired track which is a sequence of information.
The relative position of the grooves and the beam spot may be detected by use of a two-segment photodetector (2S-PD) which is composed of two photodetector segments and responsive to a reflection from a medium, as customary in the art. This approach uses the fact that the grooves diffract the reflection of a beam spot from the medium. This kind of diffraction itself has been discussed in various fields and, hence, only the result of such a phenomenon will be described herein. Specifically, a medium is provided with grooves and lands each intervening between nearby lands. When a reflected beam from the medium is incident to the 2S-PD, it produces a particular light intensity distribution on the individual photodetectors. A sum signal or groove-crossing signal is produced by combining the light current produced by the two photodetectors, while a difference signal is produced by substracting one of them from the other. The difference signal is representative of stable position information because noise components are cancelled. For this reason, the difference signal is used as a tracking error signal for positioning the beam spot relative to the grooves.
With the difference signal alone, however, it is impossible to identify the direction of the beam spot motion at a given time. Identifying the instantaneous direction of movement of the beam spot is a requisite because the position information obtained from the medium is fed back for positioning purpose. It has been customary to determine the moving direction by referencing not only the difference signal but also the sum signal, i.e., by comparing the difference and sum signals with respect to phase. A problem with this conventional scheme is that the sum signal picks up even noise because the entire quantity of reflection is detected. Especially, recorded information is superposed on the sum signal, as will be apparent from the fact that the sum signal is used to reproduce recorded information. These obstruct the accurate detection of a sum signal or groove-crossing signal representative of position information.